Method and apparatus for dynamic magnetic field neutralization

ABSTRACT

A method and apparatus for controlling the magnetic field value within a specified volume, subject to an external ambient magnetic field, wherein the method and apparatus employ a plurality of magnetic field sensors and an associated plurality of coils, together with an electrical drive circuit connecting each of the sensors to the associated coil and including a feedback coupling the output of each of said coils to all of said sensors.

FIELD OF THE INVENTION

This application is a continuation-in-part of U.S. Patent ApplicationSer. No. 345,575, filed May, 1, 1989. U.S. Patent Applications Ser. No.592,690 and 592,691 filed of even date herewith are relatedapplications.

This invention relates in general to a method and apparatus forcontrolling the magnetic field within a specified volume in an extendedambient magnetic field and more particularly to a method and apparatuswhich employ active magnetic field generators controlled by feedbackloops to compensate for the ambient magnetic field, thereby nulling themagnetic field within the specified volume.

BACKGROUND OF THE INVENTION

The need for controlling the value of the magnetic field within aspecified volume exposed to a significant ambient magnetic field, whichmay also be varying, arises in a number of situations. One suchsituation, for example, is a cathode ray tube (CRT) monitor in which theelectron beam of the cathode ray tube gun is deflected magnetically toscan with precision the phosphor face of the monitor. The precisionrequirement is even more demanding with a color screen which requireselectrons to strike the phosphor to produce specific colors by passingthrough holes in a shadow mask at precise angles to strike only theintended color phosphor. There have been a number of approaches employedto compensate for the effects of the extended ambient magnetic field inthe operation of such monitors. Of course, one straightforward approachis the use of passive magnetic shielding. However, this is limited bothas to the magnitude of field it is practical to shield, and also by theinability to surround the entire volume with shielding.

Another approach has involved the use of electrical coils positioned inlocations around the monitor, which coils are energized to produceappropriate magnetic fields. In some instances, such as that describedin U.S. Pat. No. 2,925,524 the fields so generated are preadjustedbefore the monitor is placed in operation. Another approach is describedin U.S. Pat. No. 4,380,716 in which patterns at the corners of thephosphor screen are generated by a specific portion of the electron beamoutput path. Changes in these patterns due to changes in the ambientmagnetic field are sensed by optical sensors, which in turn control thecurrent flow through the coils, providing only axial correction. Stillanother approach (described in U.S. Pat. No. 3,757,154) utilizesmagnetic sensors placed in a bridge to control the current flow throughcorrecting coils. This arrangement is, however, open loop and,accordingly cannot produce magnetic fields under positive control toaccomplish the compensation function.

It is therefore an object of the present invention to provide activemagnetic field generators, for nulling the magnetic field within aspecific volume within an ambient external field, with the energizingcurrent for the generators being controlled by the value of magneticfield measured by a number of sensors, all included in a feedbackcontrol loop.

SUMMARY OF THE INVENTION

Broadly speaking, this invention provides a method and apparatus forcontrolling the magnetic field value within a specified volume byplacing a number of magnetic field generators around the volume togetherwith magnetic sensors at specified locations and using the signals fromthe magnetic sensors in a closed loop feedback circuit affecting all ofthe generators, together with a programmable processor to maintain themagnetic field within the specified volume at the specified value. Inone example, the specified volume includes a CRT monitor surrounded by aμ metal shield and is therefore generally cubic with one open,unshielded display face. The individual magnetic field generators areformed of coils surrounding the CRT monitor within the shield and in aplane generally normal to the electron beam axis of the monitor. Thesecoils are formed and positioned to have a portion of the coil orientedaround this axis such that all the coils acting together can produce amagnetic field characterized by off axis vectors. The specifieddirection and magnitude of this vector depends upon the direction andmagnitude of current supplied to the coils. Each coil has acorresponding sensor positioned to sense a specific portion of themagnetic field near the face of the monitor. These four magnetic sensorseach have a primary axis of sensitivity and each is positioned in aplane normal to these axes, which are parallel to one another. Theposition of this sensor plane and of the individual sensors within it issuch that the output of all four sensors provides a measure of thedirection and magnitude of the external ambient field. The outputs ofthe four sensors are connected through a feedback network to all of thecoils.

The system also includes a deperming coil operated by a micro processorcontrol to provide a deperming axial field in response to specificcombinations of sensor outputs, indicating the presence of an ambientfield beyond the range of values which can be nulled by the magneticfield generators.

DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a diagram in plan view of the arrangement of a cathode raytube (CRT) within a metal shield defining a volume;

FIG. 2 is a diagram of a transverse ambient field passing through thedefined volume of FIG. 1;

FIG. 3 is a diagram of an axial ambient field passing through a definedvolume;

FIG. 4 is a diagram of the CRT and the sensors with respect to axial,horizontal and vertical directions;

FIG. 5 is a diagram of coil 1 and sensor 1;

FIG. 6 is a diagram of coil 2 and sensor 2;

FIG. 7 is a diagram of coil 3 and sensor 3;

FIG. 8 is a diagram of coil 4 and sensor 4;

FIG. 9 is a composite drawing of coils 1-4 and sensors 1-4;

FIG. 10 is a representative drawing of the four coils with the directionof the supplied currents producing an axial magnetic field;

FIG. 11 is a representative drawing of the four coils with the directionof the supplied currents producing a horizontal magnetic field;

FIG. 12 is a representative drawing of the four coils with the directionof the supplied currents producing a vertical magnetic field;

FIG. 13 is a diagram of the basic feedback control configuration; and

FIG. 14 is a schematic representation of one arrangement suitable forimplementing the feedback control configuration of FIG. 13.

FIG. 15 is a block diagrammatic illustration of a deperming circuit; and

FIG. 16 is a charging circuit for use in the deperming circuit of FIG.15.

DESCRIPTION OF THE PREFERRED EMBODIMENT System Description

The specific embodiment described is a system for nulling the externalambient magnetic field imposed upon a high resolution color monitorwhich produces a rectangular image on a shadow mask CRT 3 which hasprecision in line electron guns. The shadow mask 5 is shown in FIG. 1.These electron guns produce three electron beams accelerated to 25 Kev.The functioning of the CRT depends on precise control of magnetic fieldsalong the paths of these beams since an electron moving through amagnetic field is deflected by an acceleration proportional to the crossproduct of its own velocity vector and the local magnetic field vector.

The electron beam of the CRT scans the image area at the CRT's lightemitting phosphor screen by producing magnetic fields withelectromagnets placed near the electron guns which are at the rear ofthe CRT. Alignment of the three electron beams so that they will allconverge at the same spot on the phosphor screen also depends onmagnetic fields in the gun area. The three beams pass through a shadowmask placed approximately half an inch behind the phosphor screen. Theshadow mask has an array of small holes through it in an extendedhexagonal pattern. The center of each hole is about 0.26 mm from thecenter of its nearest neighbor holes. For each hole in the mask thereare three dots of phosphor on the CRT screen. These dots each emit adifferent color of light--red, green, or blue--when struck by highenergy electrons. When properly aligned, the beams travel through theholes at different angles so that one beam strikes only red, one onlygreen, and one only blue emitting dots. Conditions which disrupt thisalignment cause loss of color purity; i.e. the colors of the image areincorrect. The angle at which the electrons pass through the mask ortheir subsequent path can be adversely affected by stray magnetic fieldscausing loss of color purity.

The nulling system described below which is illustrated in FIGS. 1through 16 provides a neutral magnetic field in the volume occupied bythe CRT and its yoke, despite ambient magnetic fields of value up to 5oersteds, which may vary at a rate as high as 20 oersteds per second.The system of this invention neutralizes the effects of externalmagnetic fields by employing both passive and active means. A passivemagnetic shield surrounds the CRT and yoke and extends forward a fewinches beyond the CRT face. The shield is spaced about the same distancefrom the image area on the right and left sides with a slightly smallerspacing between it and the image area at the top and bottom of thescreen. Since the image area cannot be obscured, the shield is entirelyopen at the front. While the shield conforms generally to the CRT yokeoutline, it must be sufficiently separated from the yoke so that it doesnot distort the deflection fields or adversely load the circuits whichdrive the yoke. The shield may, for example, be fabricated from 80%.moly-permalloy which is typically 0.06 inches thick, except that thefront four inches of the shield may be formed with a double layer toinsure that the shield is not saturated by edge effects.

The magnetic shield is designed so that it does not saturate when placedin randomly oriented magnetic fields up to 6 oersteds strength. Thisresults in significantly reduced magnetic fields within the volume whichhave the following properties. At each point the function describing themagnetic field orientation and magnitude is determined entirely by theorientation and magnitude of the ambient vector. That function is thesame for all external field s of the same orientation except for ascalar constant which is proportional to the magnitude of the externalfield.

The active field neutralizing system employs a compensation means whichincludes a series of coils within the μ metal shield and encompassingthe CRT, each lying generally in a plane normal to the electron beamaxis of the CRT. Four magnetic sensors are positioned in a plane nearthe face of the CRT generally in each corner of it. The sensors each areoriented so that their primary axes of sensitivity are parallel to eachother and normal to the sensor plane. Each sensor output producescurrent in an associated one of the coils, as well as providing someinput signal to drive the neighboring coils. The direction and magnitudeof this current is controlled by the direction and magnitude of themagnetic field sensed by the sensor. The system is calibrated such thatthe resultant field from all four coils opposed the externally imposedambient field in both direction and magnitude, thus neutralizing itwithin the volume occupied by the CRT.

THE MAGNETIC SENSORS

FIG. 1 shows the arrangement of the primary magnetic materials int hemonitor. These materials are in a μmetal shield 1, a CRT 3, a shadowmask 5 and an internal shield 7. Although highly simplified, thisaccurately represents the situation as far as the principles to bediscussed. FIG. 1 is shown with no lines of magnetic flux present.

FIG. 2 shows lines of flux in a transverse field as they interact withthe magnetic materials of the monitor. A distance away from the magneticmaterial the lines 9 are all oriented in the same direction and areuniformly spaced. These lines and spacing represent the direction andstrength of the magnetic field. Magnetic flux has the property that itis attracted to and concentrates in objects of high permeability. Itdoes not terminate at these objects, but continues through them. In FIG.2, the flux lines to the extreme far left 9, or right 19, are uniformlydirected to the right and of uniform spacing indicating a uniformintensity magnetic field. Both the μ metal shield 1 and shadow mask 5have high permeability and attract and concentrate the flux. Anotherproperty of the interaction of flux and highly permeable material isthat the flux lines are (approximately) normal to the surface of thematerial as they enter it (lines 11 and 15) and leave it (lines 13 and17). In order to concentrate in the material, the lines of flux mustbend toward it. This results in a component of the field direction whichpoints "into" the shadow mask as it enters from the left and a componentwhich points "out" as it exits the shadow mask at the right. Thus, ifdirectional magnetic field sensors are placed symmetrically at the leftand right side of the shadow mask with their axes of sensitivitypointing into the mask, the left sensor will indicate a field of onepolarity while the sensor on the right will indicate a field of the samemagnitude but of opposite polarity.

FIG. 3 represents the situation in which a uniform axial ambient fieldhas been impressed upon the monitor. The same principles discussed aboveapply. However, the magnitude and polarity indicated by the two sensorsplaced at the left and right are the same (as opposed to oppositepolarity in the transverse field) lines of flux 21, 23, and 29 with thevolume, and lines of flux 25, 27 and 31 leave the volume.

Thus, in these two cases, the indication of the sensor at the left sideis

    V.sub.L =k.sub.a B.sub.a +k.sub.t B.sub.t

where: V_(L) is an output voltage from the sensor proportional to thefield strength directed along its axis of sensitivity, B_(a) is thestrength of the axial field in oersteds, k_(a) is a constant whichincludes the directional coupling of the axial field to the sensor,B_(t) and k_(t) are analogous for the transverse field to B_(a) andk_(a) for the axial field. The output from the sensor on the right,assuming perfect symmetry and identical sensors, is

    V.sub.R =k.sub.a B.sub.a -k.sub.t B.sub.t

since the field along the sensor axis on this side resulting from theambient field is opposite in polarity. It is now possible to separateand quantify the field components B_(a) and B_(t) since,

    B.sub.a =(V.sub.L +V.sub.R)/2k.sub.a

    B.sub.t =(V.sub.L -V.sub.R)/2k.sub.t

FIG. 4 shows a front view of the monitor with the CRT placedsymmetrically into the shield and four magnetic sensors placedsymmetrically about the horizontal 45 and vertical 47 center lines ofthe assembly. The sensors S₁, S₂, S₃, and S₄ are all placed at the samedistance from the shadow mask with their axes of sensitivity at the sameangle (nearly parallel) with respect to the CRT gun axis 33 in a plane,which includes the sensor and the gun axis. Sensors are placed as closeto the CRT image area as possible. The sensors are denoted S₁, S₂, S₃and S₄ starting at the upper left and proceeding clockwise. The outputsof the sensors are respectively V₁, V₂, V₃ and V₄. One suitable type ofsensor is a fluxgate sensor having the output signal appearing on apickup coil and being excited by a square wave applied voltage or a sinewave voltage of appropriate frequency.

Extension of the same principles used in the two dimensional analysisshows that the following expressions accurately represent the sensoroutputs.

    V.sub.1 =k.sub.a B.sub.a +k.sub.h B.sub.h +k.sub.v B.sub.v

    V.sub.2 =k.sub.a B.sub.a -k.sub.h B.sub.h +k.sub.v B.sub.v

    V.sub.3 =k.sub.a B.sub.a -k.sub.h B.sub.h -k.sub.v B.sub.v

    V.sub.4 =k.sub.a B.sub.a +k.sub.h B.sub.h -k.sub.v B.sub.v

where k_(a) and B_(a) are as above and k_(h) and B_(h) and k_(v) andB_(v) are analoqous for the horizontal and vertical ambient vectors. Theambient vectors can be determined from

    B.sub.a =(V.sub.1 +V.sub.2 +V.sub.3 +V.sub.4)/4k.sub.a

    B.sub.h =(V.sub.1 -V.sub.2 -V.sub.3 +V.sub.4)/4k.sub.h

    B.sub.v =(V.sub.1 +V.sub.2 -V.sub.3 -V.sub.4)/4k.sub.v

Thus by appropriate placement of four sensors, the three orthogonalambient field vectors are easily resolved.

The Compensation Coils

The set of four compensation coils exhibits a effective symmetry aboutthe CRT 3. Because of this symmetry, the compensation system isimplemented with a direct, one-to-one correspondence, of a singleone-axis sensor for each coil. The system uses to effective advantagethe properties of magnetic fields in the presence of magnetic materialto determine the appropriate currents to pass through each compensationcoil to null the magnetic field at each sensor. While the set of coilstogether evidences this symmetry, each coil in the set is different fromthe remainder of the coils. By an appropriate configuration of thesecoils it is possible to drive each of four coils directly from theoutput of one corresponding sensor without any additional manipulationof the sensor outputs.

A suitable construction of each of the compensation coils, for manyapplications is formed of four hundred and thirty three turns of No. 30copper wire, with each turn being approximately 72" in length. FIG. 5shows coil 55 and its associated sensor S₁, Coil 55 is nonplanar. It hasportions in each of the axial, horizontal and vertical fields. Becauseof the shape of coil 55, current flowing through the coil as shown inFIG. 5 generates a resultant magnetic field diagrammatically representedby vector 65. Vector 65 has components (59, 61 and 63) in each of thepositive axial, positive horizontal and positive vertical directionsrespectively.

FIG. 6 shows coil 71 and its associated sensor S₂. Coil 71 is nonplanar.Coil S₂ has components in each of the axial, horizontal and verticalfields. Coil 71 has the same shape as coil 55 but is rotated so that thesensor S₂ in the upper right hand corner has the same orientation to thecoil as does sensor S₁ in FIG. 5. Because of the shape of coil 71,current flowing through the coil as shown in FIG. 7 generates aresultant magnetic field diagrammatically represented by vector 67.Vector 67 has components (73, 75 and 77) in each of the positive axial,negative horizontal and positive vertical directions respectively.

FIG. 7 shows coil 83 and its associated sensor S₃ positioned in thelower right hand corner. Coil 83 is nonplanar. Coil 83 has components ineach of the axial, horizontal and vertical fields, and has the sameshape as coils 55 and 71, and again is rotated so that it has the sameorientation to sensor S₃ and coils 55 and 71 had to sensors S₁ and S₂respectively. Because of the shape of coil 83, current flowing throughthe coil as shown in FIG. 7 generates a resultant magnetic fielddiagrammatically represented by vector 79. Vector 79 has components (85,87 and 89) in each of the positive axial, negative horizontal andnegative vertical directions respectively.

FIG. 8 shows coil 103 and its associated sensor S₄ positioned in thelower left hand corner. Coil 103 is nonplanar. Its shape corresponds tothat of the other three coils and its orientation to its sensor S₄ isalso the same. Coil 103 has components in each of the axial, horizontaland vertical fields. Because of the shape of coil 103, current flowingthrough the coil generates a resultant magnetic field diagrammaticallyrepresented by vector 93. Vector 93 has components (95, 97 and 99) ineach of the positive axial, positive horizontal and neqative verticaldirections respectively.

FIG. 9 shows four essentially axial coils. Each coil includes two edgesalong the front 49, two edges along the back 57, and two cross-overs 53.

The crossovers are arranged so that no two coils are in the front on thesame two edges. The magnitude and shape of the magnetic field generatedby the set of four coils will be determined by the magnitude anddirection of currents applied to the coils in response to the sensedmagnetic field at each sensor.

Feedback Network

FIG. 13 shows a feedback network which responds to four magnetic fieldcomponents B₁, B₂, B₃ and B₄ to produce at the output of each of theassociated amplifiers 127, 129, 131, and 133 respectively, four voltagesV₁, V₂, V₃ and V₄, that drive current through the four compensationcoils C₁, C₂, C₃ and C₄. The composite magnetic field produced by thecurrent flowing through the coils produces a field at each of the foursensors of the same magnitude but opposite polarity as the ambientvectors B₁, B₂, B₃, B₄, which are aligned with the sensors. Each of thefour coils are magnetically coupled to provide a negative feedback toall four of the sensors. Thus, as any sensor detects, for example, anincrease in field strength, the amplifier output drives the associatedcoil to generate an opposing field. Not only is this field magneticallycoupled back to that sensor, by a coupling coefficient -m but it is alsomagnetically coupled through coupling coefficients identified as -n, -p,-q to the other three sensors. The negative sign indicates that themagnetic feedback is in a direction to oppose the direction of theoriginally sensed ambient field. It is important to note that, asillustrated in FIG. 14, a portion of each of the output voltages V₁though V₄ are coupled through feedback resistors R_(m), R_(n) and R_(q)to its own sensor and to the two neighboring sensors, thereby furtheradjusting (after calibration) for the contribution the magnetic fieldsgenerated by each of the coils C₁ through C₄, make at the neighboringsensors. Thus the field is nulled at all four sensors simultaneously andcompensation is achieved.

Calibration of the Coils

In order to calibrate the coils initially, an external uniform magneticfield is established over a volume considerably larger than thatoccupied by the CRT 3 and compensation mechanism. The strength of thefield should be greater than the maximum specified for the compensationsystem. For convenience the direction of the field is reversible. Alsofor convenience the field can be established exclusively along each ofthe three mutually perpendicular axes 121, 123 and 129. The monitor isplaced within the field such that the axis 121 coincides with a vectorperpendicular to the center of the image area on the CRT (along the axisof the electron beam).

Next, electric current is supplied to the compensation coils from avariable d.c. source. This is done to calibrate the effect of the coilsseparately from any feedback mechanism. The set of coils is calibratedseparately for each of the three axes. During calibration, all coilscarry the same amount of electrical current. To compensate for an axialfield only, all of the coils are connected such that they carry currentof the same polarity around the axial vector (either clockwise orcounterclockwise with respect to the front of the CRT). To compensatefor a horizontal field only, the coils are connected such that coils C₁and C₄ carry current of the same polarity and coils C₂ and C₃ carrycurrent of the opposite polarity. Compensation for a vertical field onlyis accomplished when coils C₁ and C₂ carry current of the same polarityand coils C₃ and C₄ carry current of the opposite polarity.

When an external axial ambient magnetic field is generated it produces anoticeable loss of color purity and rotation of the raster. Compensationcurrent is applied and adjusted until it is of the correct polarity andamplitude to provide the optimum restoration of color purity to the CRT3. Typically the video signal displayed on the CRT 3 will be an entireraster of a single color of constant level. This facilitatesestablishing when color purity is optimized.

The magnitude and polarity of the current for optimal compensation to anaxial ambient field is then recorded. Similarly, the currents forcompensation of horizontal and vertical ambient fields are alsoestablished and recorded. These recorded current values are very similarto one another and make up for imperfections and inequalities in the setof coils.

Feedback Calibration

Each sensor feedback factor m, n, p and q is individually optimized foreach of the three axes of ambient magnetic vectors. This is done whiledriving all four of the coils in parallel from the output of only theone sensor which is being calibrated. Since all coils are wound from thesame length of the same gauge wire, each has the same resistance andwill carry the same amount of current when they are connected inparallel across the output drive voltage generated in response to thesignal from the sensor being calibrated. Each sensor is calibrated,separately from the others and for each axis, as follows.

For calibration of sensor S₁, the four coils are connected to carrycurrent in the same direction around the axial vector and are connectedto the output drive corresponding to sensor S₁. Sensors S₂, S₃ and S₄are temporarily disconnected from the coils. When an ambient axial fieldis applied to sensor S₁, the voltage V₁ developed at the output ofamplifier 127 causes current in the compensation coils which produce amagnetic field at sensor S₁ in opposition to the ambient field. This isthe basic negative feedback mechanism of the automatic compensationsystem. The sensors are located and oriented such that, beforecalibration, the voltage V₁ produced by amplifier 127 to null the sensorS₁ has greater magnitude than is required for optimum color purityrestoration. Calibration for the axial compensation is accomplished byconnecting a variable resistor from the voltage drive to coil C₁ intothe pickup coil of sensor S₁, The other side of the pickup coil isconnected to ground. This causes a current to flow through the pickupcoil and thereby generate a local field which further opposes theambient vector at the sensor.

This situation then requires less voltage at the coil driver to null thesensor. The variable resistor is adjusted until the voltage at the coildriver matches that required for optimum axial compensation. The valueof the resistor is measured and recorded as R_(A1).

For calibration of horizontal and vertical ambient fields the procedureis repeated producing R_(H1) and R_(V1). The compensation coils arereconfigured to produce horizontal or vertical fields. The correctfeedback resistors R_(H1) and R_(V1) are determined and recorded.

The procedure is then repeated for sensors S₂, S₃ and S₄. Whenconnecting the coil set for axial correction all currents have the samepolarity. In the horizontal calibration for sensors S₁ and S₄ thecurrents in coils C₁ and C₄ are positive, while the currents in coils C₂and C₃ are negative. During horizontal calibration of sensors S₂ and S₃the currents in coils C₁ and C₄ are again positive, while current incoils C₂ and C₃ are negative. Similarly, during vertical calibration ofsensors S₁ and S₂, the currents in coils C₁ and C₂ are positive, and thecurrents in coils C₃ and C₄ are negative. During vertical calibration ofsensors S₃ and S₄ and currents in C₁ and C₂ are positive while those inC₃ and C₄ are negative.

After values of R_(AK), R_(HX), R_(VX), (X=1,2,3,4) are determined, thefollowing set of calculations is performed: ##EQU1##

The resistors R_(mx), R_(nx), R_(qx) are installed between the pickupcoil for sensor X from respectively, the coil driver for coil X, thecoil driver for the horizontal neighbor and the coil driver for thevertical neighbor. There connections are summarized in the followingtable:

    ______________________________________                                        Sensor       Coil Driver                                                      Pickup Coil  1      2          3    4                                         ______________________________________                                        1            R.sub.m1                                                                             R.sub.n1   --   R.sub.q1                                  2            R.sub.n2                                                                             R.sub.m2   R.sub.q2                                                                           --                                        3            --     R.sub.q3   R.sub.m3                                                                           R.sub.n3                                  4            R.sub.q4                                                                             --         R.sub.n4                                                                           R.sub.m4                                  ______________________________________                                    

If the calculated value of a resistor is negative, then the connectionof that resistor is made to the inverted coil driver for that sensor.

Operation

FIG. 9 depicts a composite diagram including all four sensors and coils,and shows the circuits in each coil as previously discussed. The outputof sensor 1, V₁, drives coil C₁ and similarly for sensor/coil pairs 2, 3and 4. The outputs of the sensor electronics are proportional to thetime integral of the magnetic field vector aligned with the sensor. Ifthe field is nulled at the sensor, then its output remains constant. Thedrive from each sensor output is arranged so that it produces a currentin the corresponding compensation coil which reduces the field at thatsensor. The field produced also has an influence on each of the othersensors. There is a high degree of symmetry in this interaction.

When, the system illustrated in FIG. 13 is in equilibrium, the followingcalculations are satisfied if the field at each sensor is zero.

    B.sub.1 =-mV.sub.1 -nV.sub.2 -pV.sub.3 -qV.sub.4

    B.sub.2 =-nV.sub.1 -mV.sub.2 -qV.sub.3 -pV.sub.4

    B.sub.3 =-pV.sub.1 -qV.sub.2 -mV.sub.3 -nV.sub.4

    B.sub.4 =-qV.sub.1 -PV.sub.2 -nV.sub.3 -mV.sub.4

The fact that all of the coupling coefficients are the same in eachequation is established by symmetry. The coupling coefficient of theoutput of each sensor back to itself is -m. The coupling coefficients tohorizontal neighbor and vertical neighbor are respectively -n and -q andto the opposite corner, -p.

In equilibrium the current through each compensating coil I₁, I₂, I₃ orI₄ is proportional to the sensor output voltages V₁, V₂, V₃, V₄ and canbe computed by simply dividing the corresponding voltage by the coilresistance (all coils have the same resistance, R_(c)), i.e. I₁ =V₁/R_(c), etc.

When feedback equations are solved for V₁, V₂, V₃, V₄ in terms of B₁,B₂, B₃, B₄ the following formula is obtained:

    V.sub.i =-[B.sub.i (M/D)+B.sub.h (N/D)+B.sub.j (Q/D)+B.sub.k (P/D)]

The subscripts have the following definitions: i=self; h=horizontalneighbor; k=opposite corner, and j=vertical neighbor. Thus the symbolsD, M, N, P, Q are defined as:

    D=(m+n+p+q)(m+n-p-q)(m-n-p+q)(m-n+p-q)

    M=m.sup.3 -m(n.sup.2 +p.sup.2 +q.sup.2)+2npq

    N=n.sup.3 -n(m.sup.2 +p.sup.2 +q.sup.2)+2mpq

    P=p.sup.3 -p(m.sup.2 +n.sup.2 +q.sup.2)+2mnq

    Q=q.sup.3 -q(m.sup.2 +n.sup.2 +p.sup.2)+2mnp

Based on previous discussion, the following substitutions can be made.

    B.sub.1 =B.sub.A +B.sub.H +B.sub.V

    B.sub.2 =B.sub.A -B.sub.H +B.sub.V

    B.sub.3 =B.sub.A +B.sub.H -B.sub.V

    B.sub.4 =B.sub.A +B.sub.H -B.sub.V

The quantities B_(A), B_(H), B_(V) are the strength of the component ofthe Axial, Horizontal and Vertical fields aligned with the axis ofsensitivity at sensor S₁.

This leads to

    V.sub.1 =-B.sub.A K.sub.A -B.sub.H K.sub.H -B.sub.V K.sub.V

    V.sub.2 =-B.sub.A K.sub.A +B.sub.H K.sub.H -B.sub.V K.sub.V

    V.sub.3 =-B.sub.A K.sub.A +B.sub.H K.sub.H +B.sub.V K.sub.V

    V.sub.4 =-B.sub.A K.sub.A -B.sub.H K.sub.H +B.sub.V K.sub.V

where K_(A) 32 1/(m+n+p+q); K_(H) =1/(m-n-p+q); K_(V) =1/(m+n-p-q). Fromthis, it can be recognized that the feedback system responds to theaxial component of the ambient field by supplying a current, -B_(A)K_(A) /R_(C) to each of the compensation coils. In response to thehorizontal component of the ambient field, a current, -B_(H) K_(H)/R_(C) flows in coils 1 and 4, and a current of the same magnitude butopposite polarity, B_(H) K_(H) /R_(C) flows in coils 2 and 3. Similarly,the response to a vertical field is currents, -B_(V) K_(V) /R_(C) incoils 1 and 2, and +B_(V) K_(V) /R_(C) in coils 3 and 4. Comparing theseresults to the analysis of the compensation coils leads to theconclusion that the compensation system responds to any ambient fieldwith currents in the four compensation coils that establish separateaxial, horizontal and vertical fields which exactly null the componentsof the ambient field aligned with the sensor axis at each of the foursensors.

The invention utilizes the combination of the vectors 65, 67, 79 and 93.Due to the symmetry of the set of coils 113, the horizontal, verticaland axial components of the vectors can cancel each other when summed invarious ways.

For example, when all four coils, C₁, C₂, C₃ and C₄, are driven withidentical currents, the resultant magnetic field is aligned entirelyalong the axial direction. This is because horizontal components 61, 75,87 and 97 cancel each other and vertical components 63, 77, 89 and 99cancel each other. FIG. 10 shows the resultant vector 115 which resultswhen positive currents of equal value are sent through coils C₁, C₂, C₃and C₄.

FIG. 12 shows the resultant vector 117 which results when current I₁ ispositive, current I₂ is negative, current I₃ is negative and current I₄is positive. When all currents are of equal magnitude and the abovepolarities, the resultant vector 117 is entirely in the horizontaldirection.

FIG. 12 shows the resultant vector 119 which results when current I₁ ispositive, current I₂ is positive, current I₃ is negative and current I₄is negative. When all currents are of equal magnitude and the abovepolarities, the resultant vector 119 is entirely in the verticaldirection.

The fact that the coils are all of equal size and have a common centerpoint 91 allows for the individual resultant vectors 65, 67, 79 and 93to create sum vectors 115, 117 and 119. The apparatus of the inventionallows for the magnitudes of the currents I₁, I₂, I₃ and I₄ to be equal.Other coil configurations which are centered at point 91 and createmagnetic fields similar to those represented by vectors 65, 67, 79 and93 could similarly be used.

Each of the coils is shown to be nonplanar in the preferred embodiment.However, the resultant vectors 65, 67, 79 and 93 could all be created byplanar coils centered at point 91. The planar coils would each have tobe tilted away from the horizontal and vertical axes such that theirresultant vectors would add in a similar fashion to the presentinvention. Fabricating such a set of planar coils would, however, bemore difficult because the coils would have to cross one another.Because of this the coils would have to be of varying sizes to firstwithin one another. If the coils were of varying size, then varyingcurrents would have to be sent through the coils to cause them to cancelin the fashion of the present invention. The present invention makesmore efficient use of the symmetry of the resultant vectors 65, 67, 79and 93.

FIG. 14 illustrates a specific implementation of the present embodiment.In this embodiment, the sensors employed are flux gate sensors whichemploy a specific readout coil to provide the output signal indicativeof the magnetic field at the sensor. This sensor pickup coil has a lowimpedance, typically less than 10 ohms. As indicated in FIG. 14, thenon-grounded end of sensing coil 302, which forms a part of sensor 1 ofthis coil is connected to the input of operational amplifier 304, theoutput of which is coupled to a second operational amplifier 306,configured to serve as an integrator. The output for the integrator iscoupled to an output amplifying stage including a positive driver 308and a negative driver 310 to driver the associated compensation coil C₁.The outputs of driver stages 308 and 310 are connected through threeadjustable feedback resistors R_(m1), R_(n1) and R_(q1) to thenon-grounded end of sensor pickup coils 302, 312, and 332. Similarly,the corresponding output stages for each of the other channelscorresponding to sensors S₂, S₃, and S₄, are fed back throughcorresponding adjustable resistors, to the non-grounded end of their ownsensing coils (through adjustable resistors R_(m2), R_(m3), and R_(m4))and to the non-grounded end of the sensing coils of the sensorsimmediately adjacent to them (through adjustable resistors R_(n) l,R_(n2), R_(n3), and R_(q1), R_(q2), and R_(q3)). The feedback currentsat each of the sensing coils have the effect of changing the magneticfield in a very localized area where the coil is located, so that eachsensor output is not only indicative of the general ambient magneticfield in which it is located, but is also changed by the effect of themagnetic field generated by its sensing coil in response to thesefeedback currents. Thus, as described in the calibration proceduresoutlined above, the feedback resistors can be said to provide for asymmetrical response to the ambient fields as sensed by each of thesensors, for nulling the magnetic field int he entire enclosed volume.These adjustments restore symmetry which may have been lost due toinexact physical properties of coils, sensors, and shield asconstructed.

Deperming System

A system for providing both passive (μ metal shield) neutralizing ofambient magnetic fields and active neutralizing by generating acompensation field equal in magnitude, but opposite in direction to theambient magnetic field, has been described above.

However, physical design constraints may limit the effectiveness ofpassive magnetic shielding and of compensation coils of anyconfiguration.

In the practice of a system with the presently disclosed compensationcoils and sensor configuration within a CRT, a deperm cycle is performedwhen the system is first activated. In deperming, any elements of thesystem that tend to become permanently magnetized become degaussed,removing any prior magnetization while establishing a pattern ofmagnetization within them opposing the ambient field. This isaccomplished by driving the magnetically permeable material with aninitially strong, underdamped magnetic field to saturation, alternatelyin one direction and then the other, the amplitude being graduallydiminished to zero. Thereafter the compensation coils may operate toeffectively null the ambient magnetic field within the CRT volume.However, the active nulling of the coils may not provide adequate localfield suppression in the presence of a particularly strong ambientfield. This invention, therefore includes an automatic deperming systemwhich can assist various shield, coil and sensor configurations in orderto reduce the effect of even strong or persistent ambient magneticfields within a defined volume. Thus an arrangement is provided where aninitially activated equipment, such as a CRT, is depermed and then isagain depermed whenever the magnitude of the ambient field rises outsidethe operative range of an associated active compensation coilarrangement. This subsequent deperming, in conjunction with thecontinued active compensation of the coil arrangement, is designed tocompensate for locally perturbing ambient fields to a point where theequipment operation, such as CRT image generation, is within operatingspecifications.

An illustrative embodiment of a deperm system for the present inventionis shown in FIG. 15. The deperm system operates in a volume such as thatillustrated in FIG. 1, where ambient magnetic fields are sought to benulled within a given range of specification. The deperm system includesa permanent look-up table (LUT) 204 which stores a deperm schedule, asensor output circuit 206 which typically includes a plurality of coilsand sensors as illustrated in FIG. 4 though 12 for interaction with theambient magnetic field in the equipment volume, and a deperm circuit 208which includes at least one deperm coil (not shown) for deperming thevolume containing the CRTs. The deperm coil is typically a single largecoil, for example, 80 turns of No. 10 copper wire wound around the CRTin a plane normal to the electron beam axis and positioned behind thecompensation coils. A deperm control circuit 210 receives input signalsfrom sensor output circuit 206, indicating the values of the x, y and zcomponents of the ambient field. The deperm control circuit 210 providesinformation to and receives information from LUT 204, generating acontrol output to deperm coil circuit 208 to energize the deperm coilaccording to both a pre-scheduled program and to a comparison of thesignals received from sensor output circuit 206 and the values in LUT204.

The LUT maps a plurality of operating points which may possibly beencountered within the equipment volume. A given stored operating point,for example, represents a respective value of each of the threeorthogonal x,y,z components of a sensed ambient magnetic field withinthe volume containing the CRT. The LUT stores a plurality of x,y,zoperating point sets; the breadth of this plurality of stored sets ispredetermined experimentially

The LUT also maps the acceptable range of variation around a given x,y,zoperating point which is within the nulling capability of thecompensating coils 55, 71, 83 and 103. For example, along with themagnitude of the x,y,z components which defines a particular operatingpoint, the limit values (maximum and minimum) for this point, arestored. Outside of these values the compensation coils are considered tobe incapable of efficiently nulling the local ambient field.

Initially the values of acceptable range for the x,y,z defined pointsare determined during the system calibration procedure with thedetermination of acceptable maximum and minimum departure from the fieldvalue for each defined point being selected on the basis of sensitivityof color imperfections to particular field components. In practice, thecolor imperfections are most sensitive to variations in the axial (z)field, next most sensitive to variations in the horizontal (x) field,and least sensitive to variations in the vertical (y) field.

In operation, control circuit 210 initiates a deperm cycle when theequipment is first powered-on. After completion of this deperming cycle,the x,y,z components of the ambient field in the volume are detected bycircuit 206 and are employed by control circuit 210 as a baseline set ofcomponents for a specific operating point having the values of thisbaseline set already listed within LUT 204. Control circuit 210 nowcontinues to monitor the ambient field component values detected bycircuit 206 and to compare on a continuous basis these readings to theacceptable deviation range of min/max component values associated withthe present baseline operating point as listed in the LUT. When circuit206 senses field components beyond the acceptable limits of the rangeassociated with the particular operating point, then control circuit 210temporarily holds constant the output of circuit 206, holding constantoperation of the compensation coils. It also initiates a new depermcycle, thereafter reenables circuit 206, and selects a new baselineoperating point based on the value of the field components detected bycircuit 206 after completion of the new deperm cycle. Now the controlcircuit adopts a new baseline operating point (with a new associatedmin/max range) and controls the system accordingly.

In a preferred embodiment of the invention, the equipment is a CRTdevice, and the detection and control circuit 206 includes thecoil/sensor configuration of FIGS. 4-12.

Thus, the compensation coils act to minimize the ambient field in thevolume of interest for so long as the value of the field components doesnot exceed the limits of the range within which the compensation coilsand their drive circuit are effective. This range is expressed in theLUT min/max range associated with a given operating point.

When control circuit 210 initiates a deperm cycle, a blanking command isissued from the controller to a CRT display circuit, which in turntemporarily blanks the display. But because this blanking temporarilydisrupts the viewed image, it is desirable to limit the duration of thedeperming cycle. Hence, the system is preferably provided with a "highspeed deperm circuit" which performs deperming in about 1/20 second.This limits the observed blanking action to a short blink of thedisplay.

Under conditions when automatic deperming and blinking might beconsidered to be too distracting to the observer, the system can beprovided with a capability for the observer to inhibit automatic depermaction. However, the system preferably continues to compute magneticambient field changes and may signal the operator via an on-screenindicator or a small lamp next to the display that a manually initiateddeperm cycle will improve image quality.

FIG. 16 illustrates a suitable drive circuit for the deperm coil 215.Control circuit 210 provides a deperm signal on terminal 212 whichtriggers transistor T₁, discharging capacitor 215 through deperming coilL₁. Suitable values for the components are 200 to 400 μf for capacitor215, and 5-10 mh for coil L₁. A suitable transistor T₁ is FET. S.N.IRF-250. A capacitor charging current is supplied to terminal 214 fromdeperm coil circuit 208.

It will thus been seen that the objects set forth above, among thosemade apparent from the preceding description, are efficiently attained.It will be understood that changes may be made in the aboveconstructions and in the foregoing sequences of operation withoutdeparting from the scope of the invention. It is accordingly intendedthat all matter contained in the above description or shown in theaccompanying drawings be interpreted as illustrative rather thanlimiting.

Accordingly, what is claimed is:
 1. Apparatus for controlling themagnitude and direction of a magnetic field within a specified volume inthe presence of an ambient external magnetic field comprising,aplurality of magnetic field sensors positioned in specific locationswithin said volume, a plurality of coils, each of said coils beingassociated with one of said sensors, electrical drive circuit meansconnecting each of said sensors to its associated coil and providing tothat coil electrical current which varies in magnitude and direction inaccordance with variations in magnitude and direction of the magneticfield detected by the associated one of said sensors, feedback meanscoupling the output current from each of said drive circuits to itsconnected sensor and to the sensors positioned next adjacent to thatsensor, said feedback means having individually adjustable feedbackfactors to each of said coupled sensors to provide for contribution byall of said sensors to controlling the magnetic field within saidvolume.
 2. Apparatus in accordance with claim 1 wherein a shadow maskcathode ray tube is positioned within said volume and said feedbackfactors are adjusted to optimize the operation of said cathode ray tubewhile subject to an ambient magnetic field.
 3. Apparatus in accordancewith claim 1 wherein said plurality of magnetic field sensors arearranged in a single plane in rectilinear relationship, each of saidsensors having a primary axis of sensitivity parallel to the others,said axis being normal to said plane, and wherein the next adjacentsensors to each sensor are located in the adjacent rectilinearpositions.
 4. Apparatus in accordance with claim 2 wherein said cathoderay tube is part of a color monitor and said feedback factors areadjusted to optimize color purity.
 5. Apparatus in accordance witheither one of claims 1 and 2 wherein said magnetic field sensors arefluxgate sensors having a sensing coil and wherein said feedback meanscouples to the sensing coils of said sensors providing electricalcurrent through said sensing coils.
 6. Apparatus in accordance withclaim 1 wherein the feedback factor from each drive circuit to itsconnected sensor is a negative feedback factor.
 7. Apparatus inaccordance with claim 1 wherein said coils are positioned and formed toact collectively to produce a composite magnetic field whose magnitudeand direction is controlled by the signals produced by all of said drivecircuits simultaneously.
 8. A method for controlling the magnitude anddirection of a magnetic field within a specified volume in the presenceof an ambient external magnetic field comprising the steps of,sensingthe value and direction of magnetic field at a plurality of specificlocations within said volume employing individual sensors for eachlocation, using said sensed values to energize a plurality of magneticfield creating coils, each of said coils being associated with one ofsaid individual sensors, connecting each of said individual sensors toits associated coil through an individual drive circuit to energize eachcoil, feedback coupling each of said drive circuit outputs back to itsconnected sensor and to sensors positioned next adjacent to that sensor,adjusting each feedback factor individually to provide for contributionby all of said sensors to the controlling of the magnetic field withinsaid volume.
 9. A method in accordance with claim 8 wherein said volumecontains a cathode ray tube and said feedback factors are adjusted in acalibration procedure to optimize specific characteristics of saidcathode ray tube in the presence of a magnetic field.
 10. A method inaccordance with claim 9 wherein said calibration procedure includes,first generating a uniform magnetic ambient field in a first directionincident upon said volume, then generating a second uniform magneticfield incident on said volume in a second direction orthogonal to thefirst, after removing said first field, and then generating a thirduniform magnetic field incident on said volume in a third directionorthogonal to the first two, after removing the first two magneticfields, and adjusting said feedback factors while each of said fields ispresent.